So, as the name implies, I have a lame idea, lame mostly in that I'm sure many other people have already had it, but whatever. So, for the premises of this idea, a force of vanguard Tau have come into the Expanse, and are contemplating making it part of their ever farther-flung empire, and the players, should I ever do anything with this, might be among the first people to discover their presence. So, some points, and then some questions from me to you.

Someone might be saying "how do the Tau get there?" Well, while the Tau Empire is, I believe, very far away, and their warp travel is inferior to anyone else who CAN warp travel, I am saying that somewhere, maybe in the space of the Third Sphere Expansion, maybe in the Reach (but I like that one less), the Tau discovered a stable warp gate, and while their understanding of it is a bit wanked, they did figure out what it does, and found themselves in the Koronus Expanse, possibly near Undred Undred Teef, or way out in some wilderness space, not sure. Since then, they have been slowly coming in, and learning what's up in that neck of the galaxy. Were I to run it as a game, the players would get a chance to learn of them, perhaps without the Tau's knowledge. This could be a golden opportunity to chat the Tau up, and gain some interesting trading partners, or, if it burns their Imperial sensibilities, they could try to warn the Imperium. I figure such a close xenos presence would galvanize the Imperium, at least in the Calixis Sector, to action. All of this could endanger the autonomy of many other Rogue Traders, who would have to decide where their loyalties lie. I have some thoughts on the Ethereal in charge, stationed on the cloaked station nearby the gate's terminus, and the FC Commander in charge of the military forces. In a short while, the people of the Expanse could start to see Water Caste in Footfall. As for the greater imperium, it's hard to tell what response they might launch. Jericho and the Severan Dominate each eat up numerous resources, even in that area, so they might be hard-pressed to quickly defend their borders, and parts of the Expanse aren't quite the Imperium, except in the Imperium's monologue, so they might not have the biggest care right away.

Now, for my questions:

1. In the event of boarding actions, what would be the ideal Tau set-up? In my mind, Terminators would make great boarders, but Tau Broadsides, and even Crisis Suits are big, so I was thinking of Stealth Suits, maybe occasionally led by a Shadowsun knockoff. Their tech, carried environment, and stealth tech could make for vicious boarders, maybe with Kroot support. Might there be something better? Would they have other weapons on the Suits better kitted to NOT breaching hulls?
2. I have an idea of how the various xenos of the Expanse might behave toward the Tau; Stryxis would trade, Orks would fight, Rak'Gol are douches, etc, and then there's the Eldar. Officially , do we know how these two races act toward each other, now? At one point, maybe back when the Tau seemed good in a sea of evil, the Eldar were depicted as cajoling the Tau, perhaps having helped them form, maybe looking at them as a successor race even, or something. Since then, they have done more of the "everyone sucks, forget them all" Ynnead Plan, and I don't know. Certainly, being ancient, and former rulers of everything, the Eldar would not take on the Greater Good as a creed, but would they aide the Tau, fight them, or just ignore them? Tau forces, coupled with some Farseer advisers could be truly terrifying, as the few Farseers would have many Tau soldiers to alleviate their normal quantity problems, and I don't see the Tau picking through Eldar ruins as much as Humans do. Or, they could battle, because it's 40K, if no other reason.
3. The Kroot are also an issue. Some stuff seems to say that the Kroot in the Expanse are there without the knowledge of their masters. Were the Tau to appear, would they suddenly start taking orders, giving the Tau some decent standing forces, while they muster more through the gate, or would they disappear, and try to avoid the Tau?
4. Air Caste Tau man their vessels, if I am right, but do they keep Fire Caste warriors aboard for defense and boarding actions? I don't know if their slighter frames and possibly limited movement would be hindrances, if the gravity might be weaker, hindering boarders, or if they would have Fire Caste to protect them. Might they use some sort of Crisis Suit, or are those purely Fire Caste? Sort of wish an Ethereal could go Crisis.

I think that'll cover everything for now. I'll see if anyone has stuff to say on it. Thanks for reading.

You could also run a Rogue Trader game in the Jericho Reach, trying to make a profit while a four-way war rages around you. Then the Tau are already there. The Tau could have also found a way to infiltrate the Jericho warp gate. Or you could just move the Expanse (for the purposes of your game) to a location closer to Tau space. Plenty of options for the setup. To address your questions:

1) Boarding actions are completely contrary to the Tau method of battle and so they don't use this tactic... most of the time. For very specific objectives (nab a VIP for example, or rescue an Ethereal) they may attempt such a boarding action, in which case you might see special forces and elite troops- Stealth Suits and experienced Pathfinders, supported by drones.

Kroot Auxilia make great boarders though, and Kroot Warspheres want to board so badly . Note that Kroot and Tau vessels have different modes of Warp travel, and so very rarely travel together. Warspheres are essentially migratory, traveling from habitable system to habitable system, but are capable of true warp travel- Tau vessels are not limited to only habitable systems, but are only capable of 'skimming' the Warp.

Don't worry about punching holes in the side of the ship- not even a Tau fusion gun or imperial Multi-Melta will do significant damage to the hull. Internal systems are another matter, however, and heavy melta charges can depressurize, or ignite, a compartment.

2) Eldar would treat the Tau just like they treat everyone else- pawns and potential threats to be redirected or extinguished. Eldar may advise the Tau, but that will work very similarly to Eldar advising Imperial Guard or Space Marines- an alliance of momentary convenience, nothing more.

3) The Kroot would try to hide their prior involvement in the Expanse, but then report "this is what we have found through our own scouting efforts. We await orders." So... plenty of Kroot'vesa.

4) Air caste man the ships, yes. Functionally, the Tau do not use marines- only drones. Lots and lots and lots of drones.

The major hurdle I see with it is that you have to really rewrite the established setting to make it possible, that or advance the storyline into a homebrew future where the Crusade in Jericho Reach has failed utterly and Deathwatch was unable to destroy the gate. Otherwise the Tau are on exactly the wrong side of the galaxy, even still they'd show up in the Calixis Sector first not the expanse. That is of course unless there are a second pair of gates linking the Jericho Reach and the Expanse. That might be your best option if you want to insert the Tau into the Expanse while inflicting minimal changes or creating too many contradictions with the existing official materials.

I've been pretty outspoken in the past about my dislike for the Tau as a part of the setting but I've been reading the Deathwatch Core Rulebook in preparation for playing a game of it. Anyway it seems they've been heavily retconned from what I remembered them being which was obnoxious animu meets starfleet "good guys" and have essentially been rewritten as extremely creepy totalitarian collectivists in the process of a really insidious quest to force everyone to see things their way and rub out anyone they can't convert. Honestly they're cool now, they fit the setting.

You could also run a Rogue Trader game in the Jericho Reach, trying to make a profit while a four-way war rages around you. Then the Tau are already there. The Tau could have also found a way to infiltrate the Jericho warp gate. Or you could just move the Expanse (for the purposes of your game) to a location closer to Tau space. Plenty of options for the setup. To address your questions:

1) Boarding actions are completely contrary to the Tau method of battle and so they don't use this tactic... most of the time. For very specific objectives (nab a VIP for example, or rescue an Ethereal) they may attempt such a boarding action, in which case you might see special forces and elite troops- Stealth Suits and experienced Pathfinders, supported by drones.

Kroot Auxilia make great boarders though, and Kroot Warspheres want to board so badly . Note that Kroot and Tau vessels have different modes of Warp travel, and so very rarely travel together. Warspheres are essentially migratory, traveling from habitable system to habitable system, but are capable of true warp travel- Tau vessels are not limited to only habitable systems, but are only capable of 'skimming' the Warp.

Don't worry about punching holes in the side of the ship- not even a Tau fusion gun or imperial Multi-Melta will do significant damage to the hull. Internal systems are another matter, however, and heavy melta charges can depressurize, or ignite, a compartment.

2) Eldar would treat the Tau just like they treat everyone else- pawns and potential threats to be redirected or extinguished. Eldar may advise the Tau, but that will work very similarly to Eldar advising Imperial Guard or Space Marines- an alliance of momentary convenience, nothing more.

3) The Kroot would try to hide their prior involvement in the Expanse, but then report "this is what we have found through our own scouting efforts. We await orders." So... plenty of Kroot'vesa.

4) Air caste man the ships, yes. Functionally, the Tau do not use marines- only drones. Lots and lots and lots of drones.

I have many times considered running RT in the Reach, and have numerous other postings relating to that. The issue here is that, and let's be honest, the Jericho Reach is a loss and a waste. They specifically wrote it, like the other areas, so that the "story" never need end. The tau there are, IMO, the least threat; they are purely mortal, purely of this reality, and finite. Ebongrave wastes considerable resources fighting them, and for nothing, but the Hive Fleet and the forces of Chaos are always configured so as to be unstoppable. How do I kill the Nids? One at a time, and every last one. Hardly a good war to win, and every world they touched might break out again. Chaos are as powerful as the Imperium, and they can always retreat into a place the Imperium can't follow, seemingly for free, and then come back when they are stronger. If the Necrons wake up, even somewhat lobotomized as they are, they might wipe all those folks out. I thought of the Expanse because it's a move that could be stopped, and perhaps with little change to the Expanse. This would leave many other options open to go do other quests, like before, if the Tau are defeated.

So, Tau just blow you to bits? They never go see what you've got, or take prisoners? Hmm, such a space threat, you'd think the Imperium would try harder to fight them. I can also see them have several Human ships under their sway, crews who sided with them. In this way, the Tau could move in camouflage, if you will, and ascertain the details of the area, maybe drop off some envoys, without attracting so much attention.

I can see the Eldar do that, of course, but how might the Tau see them?

The major hurdle I see with it is that you have to really rewrite the established setting to make it possible, that or advance the storyline into a homebrew future where the Crusade in Jericho Reach has failed utterly and Deathwatch was unable to destroy the gate. Otherwise the Tau are on exactly the wrong side of the galaxy, even still they'd show up in the Calixis Sector first not the expanse. That is of course unless there are a second pair of gates linking the Jericho Reach and the Expanse. That might be your best option if you want to insert the Tau into the Expanse while inflicting minimal changes or creating too many contradictions with the existing official materials.

I've been pretty outspoken in the past about my dislike for the Tau as a part of the setting but I've been reading the Deathwatch Core Rulebook in preparation for playing a game of it. Anyway it seems they've been heavily retconned from what I remembered them being which was obnoxious animu meets starfleet "good guys" and have essentially been rewritten as extremely creepy totalitarian collectivists in the process of a really insidious quest to force everyone to see things their way and rub out anyone they can't convert. Honestly they're cool now, they fit the setting.

It would be a previously unknown, unrelated warp gate; sometimes I think that the Old Ones would have made a decent number of them. In this way, they have nothing to do with DW, the Reach, or even those other Tau.

I've had mixed feelings one them, too, but I really don't like Orks, Dark Eldar, and such, and the Nids would be too hard to oust from the Koronus Expanse, once there, while the Tau, I already covered above.

Having tau in the Koronus Expanse might be very difficult, you must figure out a few things first before you decide whether or not to include them.

-How do they communicate with the rest of the empire from the other side of the galaxy?

-Are they able to use that method of travel regularly or are they stranded?

-Are they colonists or a military force?

-How do they know where in the universe they are (they barely have an idea of how big the imperium is, I don't think they would have accurate star charts to determine where they are).

-What is there objective in the Koronus expanse?

as for the local kroot in the Koronus expanse, the entire race is not allied to the Tau. There are kroot in the tau empire but they have been known to hire themselves out to humans and chaos as well so it doesn't mean that they would immediately join whatever tau forces show up.

Here is how I think a Tau expedition into the expanse would work out.

1) the tau discover the warp thing that brings them to the expanse

2) the tau send some scouting ships through it.

3) the tau lose contact with scouting ships due to range of communication (it would take thousands of years for a subspace message to reach the tau empire)

4) the tau are now stranded in the Koronus expanse with no other option they try to settle a planet and wait for reinforcements

5) the Tau in the empire, having no contact with the scout ships, decide to not send anymore ships through until they can figure out what happened.

Having tau in the Koronus Expanse might be very difficult, you must figure out a few things first before you decide whether or not to include them.

-How do they communicate with the rest of the empire from the other side of the galaxy?

-Are they able to use that method of travel regularly or are they stranded?

-Are they colonists or a military force?

-How do they know where in the universe they are (they barely have an idea of how big the imperium is, I don't think they would have accurate star charts to determine where they are).

-What is there objective in the Koronus expanse?

as for the local kroot in the Koronus expanse, the entire race is not allied to the Tau. There are kroot in the tau empire but they have been known to hire themselves out to humans and chaos as well so it doesn't mean that they would immediately join whatever tau forces show up.

Here is how I think a Tau expedition into the expanse would work out.

1) the tau discover the warp thing that brings them to the expanse

2) the tau send some scouting ships through it.

3) the tau lose contact with scouting ships due to range of communication (it would take thousands of years for a subspace message to reach the tau empire)

4) the tau are now stranded in the Koronus expanse with no other option they try to settle a planet and wait for reinforcements

5) the Tau in the empire, having no contact with the scout ships, decide to not send anymore ships through until they can figure out what happened.

Well, they figured out it was a travel path, and sent something through. I don't know if communications "aimed" through the gate could travel through it, or if communication would just be nonexistent, like a planned Commander Farsight. If worse came to worst, the gate is a warp gate, so two-way; if the Tau realized that they couldn't communicate, they just pop back through, and there are their people. The Tau side is in territory they firmly control, somewhere in their second or third sphere expansion, so they are right there, if need be. Their communication would be like anyone who ISN'T the Imperium, with their Astropaths. How do the Eldar talk at great distance? Through the Webway?

They would start out as an expeditionary force, seeing what's there, and reporting back. Finding worlds and resources, they'd start to come through. Seeing humans, ships, and more, they'd back up, and bring some military. Once they've established some stuff, they'd start trying to claim worlds, maybe going to Zayth, or Vaporious, and seeing if they would welcome the Greater Good.

As for the Kroot, I accept that some of them don't work JUST for the Tau, but the race as a whole seems to see them as worthy, and they owe them, and don't eat Tau flesh. Due to that, I don't know if Kroot so far out would still help the Tau, or hoping that the Tau are unaware of them, just hide.

I thought that the warp portal to the Koronus expanse was one way? I am pretty sure I read that somewhere which is why I thought they would be cut off. if not than yeah it could probably work so long as they are able to fight their way through the ships that are guarding it.

Also im not so sure about Tau having the ability of communicating at that great a distance, the eldar actually have covered large areas of the galaxy and thus have the technology but the tau have only explored a small segment of space and it would be unlikely that they had anything to communicate from across the galaxy since they have no frame of reference.

I thought that the warp portal to the Koronus expanse was one way? I am pretty sure I read that somewhere which is why I thought they would be cut off. if not than yeah it could probably work so long as they are able to fight their way through the ships that are guarding it.

Also im not so sure about Tau having the ability of communicating at that great a distance, the eldar actually have covered large areas of the galaxy and thus have the technology but the tau have only explored a small segment of space and it would be unlikely that they had anything to communicate from across the galaxy since they have no frame of reference.

Depends on which portal you mean. The big gate that links Koronus to Jericho is certainly two-way; it just has enough Imperial forces on both sides that they can tell you it is only one way FOR YOU. This allows them to keep threats from sneaking through into Koronus and Calixis. Your average warp gate is two-way, as is the one I made up for this hypothetical. In that way, the Tau are only a single ship through the gate away from contact with their people.

So, Tau just blow you to bits? They never go see what you've got, or take prisoners? Hmm, such a space threat, you'd think the Imperium would try harder to fight them. I can also see them have several Human ships under their sway, crews who sided with them. In this way, the Tau could move in camouflage, if you will, and ascertain the details of the area, maybe drop off some envoys, without attracting so much attention.

I can see the Eldar do that, of course, but how might the Tau see them?

Yeah, Tau pretty much just open fire. Scanners are for information gathering, but as a rule it has been determined that if the Fire Caste is involved, the information gathering phase is over. Earth Caste can examine the remains and Water Caste can re-educate survivors in the Savior Pods.

Tau have a long history of using rogue trader (as opposed to Rogue Trader) and pirate fronts, so yes.

Tau would see the Eldar as a species potentially useful to the Greater Good, but stubborn and unwilling to join, much like the gue (read: humans). They would be willing to use the Eldar as a tool (and be used by the Eldar, so long as it benefited the Greater Good), but would treat them with caution, always caution. They may also consider it to the Greater Good to cripple and capture Eldar vessels to study; the Eldar would, of course, object to this.

Regarding Tau communication, remember that the Tau Empire, as previously established, isn't exactly an empire but is actually a confederation. Each Sept has its own government and military- they just cooperate very closely with one another. Sort of like how the USA would work if the federal government was very, very weak and the states each possessed a military force, or how the Warsaw Pact worked in the face of NATO, or vice versa, during the Cold War. If a new Sept were establishing itself in the Expanse there wouldn't necessarily be a great deal of communication back. That's not even considering the possibility of Farsight enclaves establishing themselves in the Expanse rather than the Tau proper...

The Eldar are an even more loose confederation, and the craftworlds don't communicate with one another very often. When they wish too, I imagine the farseers just do the astropathic tango.

1) Boarding actions are completely contrary to the Tau method of battle and so they don't use this tactic... most of the time. For very specific objectives (nab a VIP for example, or rescue an Ethereal) they may attempt such a boarding action, in which case you might see special forces and elite troops- Stealth Suits and experienced Pathfinders, supported by drones.

Kroot Auxilia make great boarders though, and Kroot Warspheres want to board so badly . Note that Kroot and Tau vessels have different modes of Warp travel, and so very rarely travel together. Warspheres are essentially migratory, traveling from habitable system to habitable system, but are capable of true warp travel- Tau vessels are not limited to only habitable systems, but are only capable of 'skimming' the Warp.

Don't worry about punching holes in the side of the ship- not even a Tau fusion gun or imperial Multi-Melta will do significant damage to the hull. Internal systems are another matter, however, and heavy melta charges can depressurize, or ignite, a compartment.

Whilst they only engage in boarding actions reluctantly, theTau do have a specific battlesuit series developed for void and boarding engagements - it's in one of the Deathwatch supplements. The XV-46 series is somewhere between the 20- and 80- series in size, and packs close range weapons. It doesn't use the stealth systems of the 22 or 25 - they wouldn't help much in the confined, auspex-laden environment environment of a starship, but is principally a controller for masses of drones (which can be sent into ship corridors, etc, without risking the pilot) and a carrier for a fusion cutter allowing it to breach bulkheads and decks and outflank defensive strongpoints rather than fighting through them.

4) Air caste man the ships, yes. Functionally, the Tau do not use marines- only drones. Lots and lots and lots of drones.

There are occasionally fire caste crew members. The ships do have 'marine/security' contingents - fire warriors on a ship's bridge or reception area as imperial 'guests' arrive are referred to in a couple of bits of background. There's also at least one example in the background (from the Battlefleet Gothic Tau Fleet List) of the Fire Caste acting as weapons officers on a warship:

With its weapon systems crewed largely by Fire Caste warriors the Khas’a’tah has quickly developed a formidable reputation in battle. From its baptism of fire when it hunted down and destroyed no less than three Ork Onslaughts with a single salvo of missiles to its most recent encounter with the Dauntless class cruiser Jarrall’s Bane when its Mantas got inside the Imperial vessels shields and set it ablaze the Khas’a’tah has enjoyed unrivalled success. The principal reason for this is Kor’O Khas’a’tah himself, who, though a member of the air caste was raised in an orbital above Sa’cea and is therefore very familiar with the Fire Caste and their ways. His crew is an excellent example of the two castes working together for the greater good.

1. In the event of boarding actions...

What everybody else said. In addition, combining your idea with Annaamarth 's concept, their is nothing stopping your new "Sept" constructing said suits for the Air Caste members. I had a Tau NPC with a Crisis Engineer Suit design. Never used him nor finished the design, but it was a start. Of course, the Air Caste would not be as combat effective as the Fire Caste.

2. I have an idea of how the various xenos of the Expanse might behave toward the Tau...

You got the gist of it. In one of the RT books, is the list of races and their "views" toward other races. Substituting the words Human/Imperial with Tau/Great Good would fit in with almost all of those views.

3. The Kroot are also an issue....

As you mentioned, they would either help/hide/or ignore. Of course this ignore would not be a flat out rude gesture, but a more "I would help you, but all my warriors are busy with other contracts. Please come back in a year or two." Of course, just because they don't eat Tau flesh, doesn't mean they won't fight them for the right reasons. Like the one Kroot species have found a race that might curb their disease, but the Tau forces have already subjugated them. Once the Kroot start devouring the slaves subjugated race, the Tau might attack them. Whose to say the Kroot don't kill the Tau, and later say they were hired by "blah" person to capture the race. For the right $$/favor/technology, the Kroot will gladly enact revenge on "blah" person.

4. Air Caste Tau man their vessels...

As other people mentioned.

I had the concept of bring the Tau in the Expanse a while back. Except mine wasn't a warp gate that brought them. To semi-quote it (I must of been half asleep when I typed it, so I clarified some issues)

A human RT ship was transporting Tau colonists to one of their outlying planets located in the Eastern Fringe 300 years ago, when a warp storm knocked it off course. The ship was then caught in the worm hole connected to Zone 15's warp gate, which brought it into the Koronus Expanse. After 1 year or so of being adrift, it crashed landed on a semi-habitable planet where they have been able to survive with terraforming technology. They've been there for about 500 years (technically they were lost 300 years ago, but the warp is... fickle). Half the population is of humans, the other half Tau with a small mixture of Kroot. A Warsphere discovered their existence about 30 years ago, and has been entering the system every 5 years or so to trade supplies for technology.

Edit: Now that I look back at it, I guess it was the warp gate...... Odd........

Any race with a warp presence basically uses their own version of Astrotelepathy. Only the Imperium, as far as I know, radically specialize most of their psykers or rather cripple them. But even then Librarians of the Space Marine chapters can send and receive Astrotelepathic messages, just not quite as well as proper Astrotelepaths.

But then again, most astropaths are nothing more than glorfied long distance cell-phones with dubious connectivity stuck in a casket for most of their short and horrid lives. The Transcendant career in RT is an example of a very powerful psyker that probably would have become a Sanctioned Psyker instead had the proper range of their talents become more apparent before being divided into what psyker goes where.

Weakest = Food for Empy

Moderate = Astropaths

Strong = Sanctioning or "specialist training"... specialist meaning groomed for Inquisition.

Very Strong = Sanctioning or sacred bolter shell through the forehead

It was always my impression that Astropaths were on the strong side of Sanctioned Psykers, but otherwise agreed.

Well, that and that the evaluation isn't just about psychic potential but also simple willpower, but details details

Well, in the evaluation talent and willpower goes kinda hand in hand. If one imagines that those weak of will have either succumbed to their psychic potential and gone mad, dead or some other more horrible fate before they even get to Holy Terra... Cant really cite the fluff where my impression of various psychic strengths come from, 17 years of being a 40k fan means alot of fluff getting absorbed , but if one considers the situation of most astropaths...

Being locked in a casket and fed nutrients through tubes and often coming to a messy end due to strain or images to horrible to be seen, being combined into choirs to reach their potential, one can conclude that most astropaths are not very strong. The transcendants being a very special case, as they are allowed to walk around and do stuff, speaks more of their freak strength and talent among their kind.

Well, I might be wrong... But it does make sense, at least to me

And as for one topic... Tau in the Expanse. Why not, if one can explain how they got to the other side of the galaxy. Plenty of great ideas here, no need for me to repeat em

Tau can easily get to the expanse. There's a stable warp gate from there to the Jericho reach, so it'd require the entire Deathwatch setting to have been overhauled by the GM, but go for it!

Any race with a warp presence basically uses their own version of Astrotelepathy. (/snip)

So, since the Tau have pretty much no warp presence, I suppose they rely on warp-skimming courier vessels for their long-range communication needs? Assuming the Imperial presence at the Jericho Gate isn't retconned, that's going to mean that the local Tau are pretty much guaranteed to be subverting traders and pirates, or even Munitorium officials, to carry messages.

I could easily see Water Caste subverting the odd Navigator by offering some wealth and trinkets, or even subverting a shrouded or renegade Navis house by offering them a (nearly) exclusive Cold Trade route. So long as that house has Navigators supporting the war effort in the Reach, they could help pass messages. The individual navigators in the Reach may not even know for whom their messages are bound, or from whom they originate.

Underworld organizations, mercenary groups, even certain missionary factions could be subverted for the Greater Good. They could then be told to hush about it (For the Greater Good), so that the missionary factions keep treading the party line with an acceptable, if unusual, message about the truth of the God Emperor of Mankind.

A network of subverted agents and cells, careful usage of dead-drops and all the usual tricks of the spycraft trade (at which Water Caste excel), and the Tau could have a reliable, if not quick, method of communication through the Gate.

All this, of course, presupposes that there is only the one major Warp Gate in local space- while it would be unlikely to see a second warp gate in the region, 40k generally thrives on improbabilities and therefore that supposition may not be true for your game.

Any race with a warp presence basically uses their own version of Astrotelepathy. (/snip)

So, since the Tau have pretty much no warp presence, I suppose they rely on warp-skimming courier vessels for their long-range communication needs? Assuming the Imperial presence at the Jericho Gate isn't retconned, that's going to mean that the local Tau are pretty much guaranteed to be subverting traders and pirates, or even Munitorium officials, to carry messages.

As far as I know, this is exactly what they do. They do have messenger drones that travel at the same speed as everything else, meaning their empire is even more vulnerable to, say, a Tyranid attack than the Imperium since it's rare for them to get the message that a colony is under attack in time to do anything about it.

However the Tau have at least two client races that are capable of using the Warp. Kroot Shamans are Kroot that eat other psychic races enough that they do begin to develop psychic powers, and the Kroot are also able to navigate the Warp in a fashion different from the Tau. The Nicassar are also quite psychic, though I believe that power is limited to being able to psychically propel their ships around.

I'm not sure that the Tau would rely on the Kroot for astrotelepathic communication. Alien minds are alien, and this is very true for the Kroot. The Tau know this, and have to account for it regularly, and would wish to avoid mistranslated messages.

The Tau ability to use to use drones might actually help them get messages out in the case of Tyrannic incursion, just because it's harder to stop fourteen drones than it is to stop one or two ships- and the Shadow in the Warp means precisely ****-all to the Tau.

But that's a bit off-topic... unless OP wants to use the Tyranids as well.

Nope, like I said, I like the Tau, in this case, because they are still people; weasely, sterilizing Commie people, but still with character, and personality, and they show up in numbers that can be defeated. Once Nids show up, you might think to just right the sector off. Granted, this didn't stop me from suggesting a Genestealer Cult aboard a Rogue Trader's ship, "seeding" the cause on various worlds they might frequent, but it can be along span between starting Cult and arriving Hive Fleet. Nids are just something to fight, with no dialogue, little gain for Rogue Traders, and on. Tau can be a source for tech, willing or no, info, and conflict.

If nothing else, several of the "fun" NPCs I have devised of late seem to be Tau Commanders, and while they would be fine for something like Deathwatch, were I to get to a point where I was running a 40K game, it's more likely they'd want to play Rogue Trader, for a few reasons, good and bad. Not sure if Commander Coldsteel, Commander Fisto (name in work), or Commander Winged Death would really fit well with space actions, anyway, but I could see, in my head, Stealth Suits and Kroot being very capable boarding teams, and with Imperial tech being junk, those auspex would probably not find anything, if they were even still operable.

Don't know, not a big fan of Orks or Dark Eldar, and most of the other races in the Expanse are one offs, weird aliens made for there that don't have a reflection in the TT, which is where much of my education came from. I love Eldar, but am not so keen on them always being the bad guys; hell, that's also part of why I was asking about a place a Rogue Trader and a Corsair Captain could sit down and share drinks, were they both so inclined.

Tau don't just have to show up during space actions. They might be setting up outposts and whatnot.

You know what I might find intriguing? Water caste agents. Water Caste doesn't do combat, ordinarily, but they do seem best suited for handling Intelligence beyond the battlefield. So, might they recruit and train human agents, individuals who have shown great loyalty to the Greater Good (or well received the indoctrination)?

Eldar in 40k are, in my opinion, very interesting, because they aren't always the "good guys," nor are they always the "bad guys." Hell, they aren't even exactly people, but more an outstanding incarnation of alien minds being alien.

They don't go out for drinks, because they can't afford to indulge themselves. Those corsairs who do indulge themselves to a degree even approaching the most austere of Rogue Traders become either playthings of Slaanesh or Dark Eldar.

They exist under a high doom and the race as a whole uses the path system to cheat that fate as long as possible, but they still get a few individualistic types who go out and join the corsair bands, putting their fates at risk.

An Eldar corsair could certainly be on something resembling amicable terms with a Rogue Trader, but that wouldn't prevent the corsair from butchering the RTs entire crew the moment it was moderately profitable to do so.

So, since the Tau have pretty much no warp presence, I suppose they rely on warp-skimming courier vessels for their long-range communication needs? Assuming the Imperial presence at the Jericho Gate isn't retconned, that's going to mean that the local Tau are pretty much guaranteed to be subverting traders and pirates, or even Munitorium officials, to carry messages.

It's called the Skether'Qan (Messenger) class. By imperial standards it's a small raider, lightly armed but with decent ranged weapons, and slightly faster than imperial civilian ships (i.e. quite a cow by Navy standards).

Most impressively, though, the thing is laced with massive drone AI support and supposedly requires only a single pilot.

Obviously this is essentially just to jump from relay station A to relay station B, download messages then turn the ship over to Air and Earth Caste techs for maintenance, but it's still impressive for a kilometre-long ship.